In recent years, hybrid vehicles have become popular because of the social demand for low fuel consumption and low emission vehicles. In a hybrid vehicle, a drive power of the vehicle is procured from both of an engine and a motor-generator, and the engine in the hybrid vehicle is also used to charge batteries, that is, for charging a main battery by driving the motor-generator to generate electricity. Further, a heater device, or an electric heater, in the hybrid vehicle is operated by receiving an electric power either from the main battery or from a sub-battery that is electrically connected to the main battery, for heating a vehicle compartment or the like.
In the hybrid vehicle having an electric heater for heating operation, for example, the heater device disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. 2013-18420) stops an electricity supply from the main battery to the sub-battery when a main battery SOC is equal to or lower than a preset value, so that a heating capacity of the heater device as well as an EV travel capacity of the hybrid vehicle are preserved without consuming the electric power in the main battery.
The technique in the patent document 1 may cause a run-down of the sub-battery when a low-SOC state of the main battery continues for a long time, due to a no-charge control of the sub-battery.
In view of such a shortcoming of the conventional technique, an idea of the present disclosure is devised.